This invention relates to a method of and a device for measuring dimensions of a workpiece in coordinate positioning apparatus. A typical such apparatus comprises a probe for sensing a surface of the workpiece, means for moving the probe relative to the workpiece so that the probe may be moved to any selected surface thereof, means operative during movement of the probe for continually measuring the position of the probe relative to a datum, and means for determining the instantaneous position of the probe responsive to a signal output by the probe when sensing said surface. Such apparatus is used for so-called universal measuring, that is, it is used for measuring workpieces of different shapes and sizes, and usually also for measuring internal as well as external dimensions. Universal measuring requires the probe to be versatile in the sense of being able to sense surfaces to a significant extent regardless of their orientation. Also, it is often a requirement to move the probe from one surface to the next under automatic control and at great speed.
A known probe senses a surface by engaging it with a mechanical stylus. The stylus has a spherical end which, by viture of its spherical shape, can readily engage a surface lying at any angle, within a relatively large range, relative to the stylus. Therefore, for many typical operations it is not necessary to change the orientation of the stylus to suit the orientation of the surface. However, when the mechancial stylus is moved towards a first surface to be measured, the stylus must be halted substantially when it engages the first surface. Thereafter the stylus must be withdrawn from the first surface and, when determining the route of the stylus to the next surface to be measured, care must be taken for the stylus to avoid any obstructing projection defined by the first surface and an adjacent second surface lying at an angle thereto. The avoidance of such projections tends to slow down the operation of the apparatus especially when it is desired to make a large number of measurements automatically and at high speed.